1. Field of the Invention
The present invention relates to a motor and, more particularly, to an apparatus and method for controlling torque of a motor.
2. Description of the Background Art
In general, a conventional apparatus and method for controlling a torque of a motor controls a torque of a motor in such a manner that a voltage value obtained by multiplying a ratio between a rated voltage simply supplied to a motor scalarly and a rated frequency by a speed instruction value expressed as a frequency and a voltage drop value according to a resistance of a stator of the motor are added, and a voltage corresponding to the added value is outputted to the motor to thereby control a torque of the motor.
The voltage drop value is obtained by multiplying a stator resistance value of the motor and a value of a current flowing at the stator resistance. However, with this control method, motor starting is not easy according to an operation form such as a speed of the motor or a load capacity applied to the motor.
The apparatus and method for controlling a torque of a motor in accordance with the conventional art will now be described with reference to FIGS. 1 to 3C.
FIG. 1 illustrates an equivalent circuit of a general induction motor.
As shown in FIG. 1, the equivalent circuit of the induction motor includes a resistance (rs) and inductors (XIs, Xm) constituting a voltage loop for an input voltage Vas; and inductors (XIr, Xm) and a resistance (rr/S) constituting a voltage loop for an output voltage (Vas/S).
That is, when a speed of the induction motor is controlled by making a radio between the input voltage (Vas) and a frequency (F) to be constant, the resistance (rs) has a constant value because it is a parameter irrespective of the frequency, whereas inductance of the inductors (Xm, XIs) is varied in proportion to the frequency. At this time, the relation between the inductance and the frequency is defined by the following equation (1):
XIs=2xcfx80xc3x97Fxc3x97IIs,Xm=2xcfx80xc3x97Fxc3x97Imxe2x80x83xe2x80x83(1)
wherein IIs and Im are values of current flowing at each inductor (XIs, Xm), and xe2x80x98Fxe2x80x99 is a frequency.
As noted in equation (1), when the ratio between voltage and frequency is made constant and the frequency is lowered down, a voltage (Eas) applied to the inductor (Xm) is reduced and an excitation current flowing at the inductor (Xm) is also reduced, so that an output torque of the induction motor is reduced. The output torque (T) of the induction motor is defined by the following equation (2):
T=kxc3x97Imxc3x97Itxe2x80x83xe2x80x83(2)
wherein xe2x80x98kxe2x80x99 is a constant, Im is an excitation current value, and It is a torque component current value generating a torque.
In case of driving the induction motor by using the defined output torque (T), a torque should be suitably controlled according to conditions of a load applied to the induction motor.
For instance, when an elevator is full with passengers in a stop state, if a motor is initiated for an ascending operation, a torque boost voltage should be additionally applied to the motor in order to output more torque than the torque as shown in equation (2) because much torque is necessary at the initial stage of starting. In this respect, however, if the torque boost voltage is increased more than necessary, an over excitation current is generated to damage the motor and the inverter electrically connected to the motor may be damaged due to the overcurrent.
On the other hand, if the torque boost voltage is in short supply, the output torque of the motor is not enough to initiate the motor and ascent the elevator.
At this time, in case that a standard motor is operated by an inverter, since a voltage (V) is varied in proportion to the variation of the output frequency (F), a voltage drop is much increased in a low frequency domain and a torque generated from the motor is very small compared to a torque of a commercial electric power supply. Therefore, a voltage for compensating the shortage of the output torque of the motor by increasing voltage suitable to the voltage drop in the low frequency domain is called the torque boost voltage.
The motor torque control apparatus in accordance with the conventional art will now be described with reference to FIG. 2.
As shown in FIG. 2, the conventional motor torque control apparatus includes: an angular velocity calculation unit 1 for calculating a radian frequency (We) on the basis of a goal frequency (in other words an output frequency) (F*) corresponding to a speed instruction value; a V/F converter 2 for receiving the previously stored goal frequency (F*) from a memory unit or from user""s setting, converting the goal frequency (F*) into a voltage instruction value (V*) according to predetermined ratio (V/F) between an input frequency (F) and an input voltage (V) and outputting it; a voltage drop calculation unit 3 for receiving a current (Ias) flowing on a stator winding (referred to as xe2x80x98statorxe2x80x99) of the motor 6 from the previously stored memory unit or from a user""s setting, and multiplying the inputted current value (Ias) and a stator resistance value (Rs) of the motor 6 to calculate a voltage drop value; an adder 4 for adding the voltage instruction value (V*) from the V/F converter 2 and the voltage drop value from the voltage drop calculation unit 3 and outputting it; and an inverter 5 for controlling an operation of the motor 6 according to the a radian frequency (We) and a voltage generated from the adder 4.
That is, the conventional motor torque control apparatus is constructed such that the voltage obtained by converting the goal frequency (F*) into the voltage instruction value (V*) by means of the V/F converter 2 is compensated with a voltage drop component according to the stator resistance of the motor 6 and the compensated voltage is applied to the inverter 5, thereby controlling the torque of the motor 6.
FIGS. 3A to 3C are graphs showing a V/F profile that a boosted instructing voltage is outputted to the motor according to a load applied to the motor and an operation direction in accordance with the conventional art.
For instance, if a rated voltage of a motor is 220V and a rated frequency is 50 Herz, a voltage-to-frequency ratio is 220/60, which is about 3.7. the conventional motor torque control apparatus supplies a instructing voltage to the motor through an inverter (in more detail, a pulse width modulator and switching devices) according to the voltage-to-frequency ratio.
FIG. 3A is a graph of a voltage-to-frequency profile applied to a conveyer or a small freight car driven by electricity, showing comparison between a torque boost quantity and an output voltage according to a certain voltage-to-frequency ratio. The conveyer or the electric-driven freight car refers to a load making a forward movement or a reverse movement in a horizontal direction, and when it moves forwardly or backwardly, the same torque is necessary. Thus, the conventional motor torque control apparatus outputs a instructing voltage according to a predetermined voltage-to-frequency ratio until it reaches a rated frequency of the motor equally in the forward movement and reverse movement of the load, which is the profile as shown by dotted lines in FIG. 3A.
However, because much torque is required in the initial starting (that is, in case of the low frequency with a low speed), actually, a instructing voltage like a profile as shown by a solid line in FIG. 3A is boosted and outputted, and when the motor is rotated with a frequency close to the rated frequency, that is, when the speed of the load approaches the rated speed, the boost voltage becomes small, and then when the motor reaches the rated frequency, the boost voltage becomes zero.
FIGS. 3B and 3C are graphs of a voltage-to-frequency profile for an ascending/descending load such as the elevator or a hoist, showing comparison between a torque boost quantity and an output voltage according to a predetermined voltage-to-frequency ratio.
In case of the ascending/descending load, much torque is not necessary for starting for operation in a descending direction, so that a torque boost of the motor, that is, a boost voltage, is not necessary.
Therefore, in case that a rotational direction of the motor to drive the ascending/descending load in the ascending direction is a forward direction, when the ascending/descending load ascends, a boost voltage greater than an output voltage according to a predetermined voltage-to-frequency ratio in the low frequency is added to output a instructing voltage, of which a voltage-to-frequency profile is like the upper profile in FIG. 3B. And, in case that a rotational direction of the motor to drive the ascending/descending load in the descending direction is a reverse direction, when the ascending/descending load descends, a boost voltage is not necessary, so that an output voltage is controlled by the output voltage according to the predetermined voltage-to-frequency ratio like the lower profile in FIG. 3B.
Meanwhile, when the rotational direction of the motor for driving the ascending/descending load in the ascending direction is a reverse direction, when the ascending/descending load ascends, a boost voltage greater than the output voltage according to the certain voltage-to-frequency ratio in the low frequency is added to output a instructing voltage, of which the voltage-to-frequency profile is like the upper profile of FIG. 3C. And when the ascending/descending load descends, that is, when the motor is driven in the forward direction, a boost voltage is not necessary, so that an output voltage is controlled by the output voltage according to the certain voltage-to-frequency ratio like the lower profile in FIG. 3C.
The conventional motor torque control method is a method for supplying an output voltage (V) calculated by the below equation (3), which rails to control constantly an excitation current of the current:
Output voltage (V)=Rsxc3x97Is+(rated voltage/rated frequency)xc3x97command frequencyxe2x80x83xe2x80x83(3)
wherein Rs is a stator resistance value of the motor and Is is a value of current flowing at the stator resistance of the motor.
That is, in the conventional motor torque control method, motor torque is controlled only on the basis of a voltage obtained by compensating a voltage corresponding to the rated voltage-to-rated frequency ratio with a voltage drop value according to the current flowing at the stator resistance.
Thus, as shown in FIGS. 3A to 3C, even if torque is controlled with a voltage obtained by adding the boost voltage to the output voltage, an excitation current of the actual motor can be excessive or in short supply. Therefore, the motor may be damaged by the excessive excitation current or a desired torque amount may not be generated due to the shortage of the excitation current.
In addition, in the conventional motor torque control apparatus and method, if a load capacity applied to the motor is greater than a motor torque according to the voltage command according to a predetermined voltage-to-frequency ratio or greater than a motor torque according to a boosted voltage command, the motor can not be started. That is, if the load applied to the motor is large, the motor is not started but simply vibrated by being rotated in the forward direction and in the reverse direction, not the direction desired by the motor,
Moreover, in the conventional motor torque control apparatus and method, if the stator resistance value is inaccurate, it is not possible to accurately control a torque of the motor, for which, thus, a desired accurate stator resistance value should be measured and set.
Therefore, an object of the present invention is to provide a motor torque control apparatus and method that are capable of generating an optimum torque boost voltage regardless of a load capacity applied to the motor, and precisely and stably controlling a torque of the motor on the basis of the generated optimum boost voltage.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a motor torque control apparatus comprising: a frequency-to-voltage converter for converting a command frequency to a voltage value according to a predetermined rated voltage-to-rated frequency ratio of a motor and outputting it; a magnetic flux controller for outputting a compensation voltage to compensate a difference between a predetermined reference value of a magnetic flux component current and a measured actual magnetic flux component current value; a stator voltage drop calculator for receiving the measured actual torque component current value, calculating a voltage for compensating a voltage drop by the stator resistance of the motor and outputting it; an adder for summing output values of the magnetic flux controller, of the frequency-to-voltage converter and of the stator voltage drop calculator and outputting the summed value as a torque component command voltage; a magnetic flux component command voltage generator for receiving the value of the measured actual magnetic flux component current and the value of the actual torque component current, calculating and outputting a magnetic flux component command voltage; and an inverter for controlling the motor according to the torque component command voltage received from the adder and the magnetic flux component command voltage received from the magnetic flux component command voltage generator.
To achieve the above object, there is also provided a motor torque control method comprising the steps of: measuring a value of actual magnetic flux component current flowing at a motor; generating a compensation voltage for compensating a difference value between the measured actual magnetic component current value and a predetermined reference magnetic flux component current value; converting a command frequency into a corresponding voltage according to a predetermined ratio between a rated frequency and a rated voltage of the motor; calculating a voltage drop value according to a stator resistance of the motor by multiplying a stator resistance value of the motor by the measured actual torque component current value; summing a value of compensation voltage of the difference value, a value of voltage converted from the command frequency, and the voltage drop value, and generating a torque component command voltage value; generating a magnetic flux component command voltage value on the basis of the measured value of actual torque component current flowing at the motor, the measured value of actual magnetic flux component current flowing at the motor, the stator resistance value, an angular velocity obtained by multiplying the command frequency by 2xcfx80 and a leakage reactance of the motor; and controlling a torque of the motor according to the torque component command voltage value and the magnetic flux component command voltage value.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.